I've successfully upgraded to Drupal 6.1.

The upgrade to Drupal 6.0 went fine, although I missed out the step about disabling contributed modules before the upgrade. So I had about an extra 30 minutes going through sorting out the results of my mistake.

The only other real issue I had was to do with the Path Auto module. It requires an upgrade to the Token module as well, otherwise you'll see errors about Path Auto not being compatible with Drupal 6.x. Update the Token module and it runs more happily.

I've now just finished the upgrade to Drupal 6.1. Took all of 30 minutes.

I'm still left with a core theme and I'm missing a couple of non-essential modules for the time being.